1. Field of the Invention
This invention relates to a multi-layer optical disc having its recording portion made up of plural information recording layers, and a recording/reproducing apparatus.
2. Description of the Related Art
Recently, with the advent of the so-called multi-media in the optical recording mediums, there is an increasing demand for handling the large-capacity information, such as digital moving pictures, such that increasing necessity is felt to store the large-capacity information to random-access and record/reproduce the stored information.
Among these random-accessible recording mediums, there is such an optical recording medium of large capacity that can be taken out or removed from the recording/reproducing apparatus. This type of the optical recording medium has so far been used in large quantities in various fields of application.
Under this situation, there is proposed, as a next-generation optical recording medium, such an optical recording medium which is able to record/reproduce for four hours on its one surface in accordance with the NTSC (National Television System Committee) system.
With this optical recording medium, it is targeted to enable recording/reproduction for four hours, as a video disc recorder for household use, in order to provide the function as a new recording medium which takes the place of the currently dominating video tape recorder (VTR). Moreover, since the optical recording medium has the same shape and size as the digital audio disc having the musical data recorded thereon, the optical recording medium is likely to be accepted by the user accustomed to the digital audio disc.
Meanwhile, the above-mentioned optical recording medium tends to be demanded to handle a larger quantity of the information, such that a larger recording capacity than is available at the present time is a desideratum.
For example, in a recording medium adapted for recording/reproduction under the above-mentioned NTSC system, it is currently desired to design the recording medium in a disc shape to exploit the speed in accessing characteristic of the disc to provide a small-sized easy-to-handle recording medium of large recording capacity in which it is possible to provide the diversified functions such as instantaneous recording/reproduction, trick play or editing.
However, in order to provided these diversified functions, the recording medium is required to have a recording capacity not less than, for example, 8 GB. The optical recording medium, that is able to realize this large recording capacity, is lacking at present for the following reason.
In a replay-only DVD (digital versatile disc), currently proposed, only the recording capacity of 4.7 GB is available, with the wavelength .lambda. being 0.65 .mu.m and the numerical aperture NA of the optical system being 0.6.
Therefore, in order to provide the recording capacity of not less than 8 GB with the signals format, such as the ECC (error correction code) or the modulation system, of the DVD system, the following relation (1) EQU 4.7.times.(0.65/0.60.times.NA/.lambda.).sup.2.gtoreq.8 (1)
From this equation (1), it is necessary that NA/.lambda..gtoreq.1.20, meaning that a shorter wavelength or a higher NA is required.
If the higher NA is achieved, it becomes necessary to reduce the thickness of a transparent substrate of the optical disc through which is transmitted the illuminated reproducing light. The reason is that, with increase in NA, the allowance for the aberration produced due to the angle by which the disc surface is deviated relative to the optical axis of the optical pickup, that is the so-called tilt angle, becomes smaller, the aberration ascribable to this tilt angle becoming the larger the thicker the thickness of the transparent substrate through which is transmitted the reproducing light.
For the similar reason, the fluctuations of the thickness of the transparent substrate, through which is transmitted the reproducing light, need to be reduced to a pre-set range.
As a transparent substrate of the optical recording medium, such a substrate produced on injection molding of plastics is predominantly used. However, this injection-molded substrate is difficult to produce within narrow tolerance to an extremely thin thickness.
On the other hand, the recording density in the in-plane direction, that is in a two-dimensional direction, of the information recording medium of a optical recording medium, depends on the minimum spot diameter of the laser light used, so that, the smaller the minimum spot diameter, the higher is the recording density that can be achieved in the signal recording. Therefore, in order to enable high recording density for the optical recording medium, attempts are currently made towards reducing the wavelength of the light source or increasing the numerical aperture NA of the objective lens. However, there is imposed a technical limitation on decreasing the wavelength of the laser light or increasing the numerical aperture NA of the objective lens, such that the recording density in the two-dimensional direction is, at it were, approaching its limit value.
As discussed in the foregoing, a higher recording capacity in an optical recording medium is becoming one of the targets in the technical investigations.